PostBlock: Implement New XKit migration

[marcustyphoon]

Description

#1279 but for PostBlock.

Along with new-xkit/XKit#2182, this implements a button that migrates PostBlock data from New XKit to XKit Rewritten when pressed. It communicates via custom events from one extension's content script to the other.

Testing steps

• Load the corresponding New XKit PR (one can check out the branch before installing PostBlock or can force update PostBlock).
• Use New XKit to block at least one post.
• Enable PostBlock in XKit Rewritten.
• Press the button in the New XKit PostBlock settings pane and confirm the migration.
• Unblock the post in New XKit. Refresh the page. (Apparently we never added cleanup to XKit 7 PostBlock, whee).
• Confirm that the post is hidden, and that it appears again when the last entry is removed from XKit Rewritten PostBlock.